FIG. 1 shows a block diagram of an electrical generator system comprising a turbine generator 10 for supplying electrical power to an electrical grid 12 via a frequency converter 14. The turbine generator comprises a turbine 16 coupled to an electrical generator 18. The turbine 16 is driven by a fluid, typically air or water, the specific construction of the turbine 16 typically depending at least in part on the driving fluid. The most common types of turbine generators 10 are driven by wind or by tidal streams/currents.
In use, the turbine 16 drives the generator 18 to produce AC electrical power by means of an AC electrical output signal. Most turbines are operable with a variable rotor speed and so variations in wind speed or tidal flow rate can cause a corresponding variation in the frequency of the generator output signal. The frequency converter 14 stablizes the frequency of the output signal to compsensate for wind or tidal flow variations. In particular, the frequency converter 14 adapts the frequency of the generator output signal to suit the frequency requirements of the grid 12.
However, the variation in fluid flow rate can be substantial—for example for a typical wind or tidal turbine generator fluid velocity can vary up to 40% about a mean value—and this can cause problems relating to voltage and power control.
Variations in the rotational speed of the turbine 16 can cause a corresponding variation in the voltage level produced by the generator 18, particularly in the case where the generator 18 is a permanent magnet generator. In many cases, the pitch of the turbine blades is fixed, which exacerbates this problem. For turbines that have variable pitch rotor blades, pitch control can compensate for turbulence but at the expense of wear on the pitch mechanism and the requirement of a fast-acting control system.
One solution to this problem is to use a frequency converter that has the capacity to withstand the highest anticipated temporary high voltage. However this is undesirable for reasons of cost. Another solution is to isolate the frequency converter in the event of extreme voltage excursions from the generator to avoid damage to the input stage of the converter. However, this creates an undesirable interruption in supply to the grid 12 and creates a further problem of how to manage re-connection of the converter.
A further issue is that variations in flow velocity cause variations in the power of the generator output. In particular, it may be seen that output power of the turbine generator 10 varies with flow velocity cubed. Extra energy is associated with turbulence because for every short interval, ΔT, when the flow velocity is higher than the mean, Vmean+δ, there is a corresponding period when the flow velocity is lower to the same extent, Vmean−δ. The energy delivered during those two intervals is proportional to ΔT·(Vmean+δ)3+ΔT·(Vmean−δ)3, which is equal to 2·ΔT·V3mean·{1+3·(δ/Vmean)2} and typically represents 4-5% additional power compared with a steady flow. This may be regarded as an opportunity not a problem but action is required to benefit from the opportunity. One option is to keep the converter connected during the period of higher flow and transmitting the maximum power during that period. However, that would involve having a very highly rated converter.
Fluctuating power is not desirable for operation of the grid 12, although it is less of a problem for a turbine farm where the outputs of multiple turbine generators are aggregated before supply to the grid than it is for instances where a singe generator, or a small number of generators, are connected to the grid 12.
It would be desirable to provide a turbine generator system mitigating the above problems.